Hello, Jeff!
My code is something like this:
import arcpy.mapping as MP
lyrboias = MP.Layer("c:/GIS_tutorial/SHP/BOIAS_PAC.shp") 
lyrboias.name = "Boias" 
molde_boias = MP.Layer("c:/GIS/Boias_molde.lyr")
MP.AddLayer(df, lyrboias, "TOP") 
updateLayer = MP.ListLayers(mxd, "Boias", df)[0] 
MP.UpdateLayer(df, updateLayer, molde_boias, False)
However, I noticed that it is, in fact, updating the "molde_boias". The problem is: in my shape called lyrboias I have an item in the attribute table called "Tipo". I need it to classify the SHP in the two classes I have inside the "Tipo" attribute and collered it with green and red. 
Thank you for your help!
Letícia